2010 Mercury 250 PRO XS burning #6 cylinder HELP

SN:1B787856

Backstory: I have a 2010 250 pro xs i bought used, running fine but the engine had 95psi across all 6 cylinders, i decided to rebuild it locally and after 4 hours of runtime it melted #6 piston cruising at 4500rpm. We took it apart again, i had all the injectors checked and two air injectors were suspicious, so i bought two new used injectors, had them checked and installed them in random locations on the engine. After the second rebuild, the engine ran for 9 hours flawlessly until, again, it burned #6 cylinder. On the rebuild we rectified the cylinders, changed all 6 pistons (pro marine brand) and changed the #6 Rod(pro marine brand). I noticed while running, sometimes the RPM would jump up 200ish rpm randomly and then settle back down to the rpm i selected after about 3-5 seconds.

@Don i’m the person you spoke to on 05/23 midday. Attached are the pictures you requested.

How do the rod and wrist-pin bearings look on that cylinder?

In an effort to help others avoid this significant failure, I am hoping to understand what happened before the piston melted and perhaps what can be done to catch the problem and shut the motor down before it gets to that.

Question: I would think the mercury high temp alarm would sound?

As a back up engine temp gauge, I purchased https://www.siliconmarine.com/ with the bolt mount $150/twins. This gauge is crazy accurate and has a user defined high temp alarm setting so you can set it as carefully as one wishes. Attachment 523584

I might be paranoid but we all know temperature kills motors and so .....

I doubt seriously that would have helped or even given you a slight indication of this failure. #6 is the COLDEST cylinder on the engine.

If it were a high temp scenario, you'd have melted down multiple other cylinders FIRST.

Interested in seeing the needles and rod-ends for that cylinder (also how well oil was distributed on those components, compared to #5).

That cylinder sure "looks" dry in the photos.

What would the effect of the check valve sticking open in that cylinder.

I will get the pictures saturday when my work week is over and i’m able to head to the shop. Overheating is out of the question I have my analog plus my mercmoniter gauge that i keep a close eye on always. And my horn is working as it should(tested with DDT).
Another thing to note is that before and after each rebuild we tested the engine with the auto self test and it all checked out perfect.

When i first pulled it apart, the cylinder did look slightly dry(oil) compared to the cylinder next to it. When i pulled the intake off there was oil on all the little pockets under the intake. We blew air thru the line that goes from there to the cyl to check for blockage and little to no oil came out. When the engine blew the first time we ran a prime sequence and i had oil coming out of both lines evenly on the 4/6 cyl. I’m going to check that again on saturday when i’m able to go back. Also i’m going to check for blockage on all the oil lines just in case.

Anything else that you guys suggest I check while i’m there ?

seems i remember an engine DON was looking at that lost a cyl where he found a sliver of metal lodged in fitting on plenum ,,, new oil pump , new direct injector , and new fuel injector

Quote:

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Originally Posted by JOE54

seems i remember an engine DON was looking at that lost a cyl where he found a sliver of metal lodged in fitting on plenum ,,, new oil pump , new direct injector , and new fuel injector

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+1

There was a cracked plunger in the oil pump, one little fragment/shard of brass ended up in the 90 degree fitting at the reed plate and restricted oil flow to that cylinder (which failed). Crank and rod were ok, piston/cylinder were not.

I blew the lines back from the cylinder to the plenum, but i will check them with a flashlight when i go back on saturday. I am also going to change all the oil lines, any idea where i can get them ?

Could the oil pump be failing to provide oil to just one cylinder or an oil pump failure would affect all cylinders?

Possible, but unlikely unless plunger o-rings are bad. Now if there is a restriction somewhere, anything is possible.

Attached are the pictures of #6 piston, piston pin, needle bearings of the piston, and the cages and roller bearings of the crank on the #6.

The bearings were wet with oil when i removed them.
Attachment 523710Attachment 523711Attachment 523712Attachment 523713Attachment 523714Attachment 523715Attachment 523716Attachment 523717Attachment 523718Attachment 523719Attachment 523720Attachment 523721Attachment 523722
Note: the piston pin was HARD to remove, i had to smack it with an aluminum punch to remove the pin. Also, the shavings on the bearings were from the table i was working on.

I also checked the check valve and line from the pump to the #6 reed, nothing out of the ordinary except the line was slightly higher on the pump fitting(see pic). I removed check valve and checked for blockage and noticed i couldn’t get it to flow with my mouth unless i sucked on the NPT side that goes screwed into the reed plate.

just dont look oil related to me , suggest you send both the inj for that cyl to DON for testing

I pulled them off i’m gunna have them checked again locally just incase.
For clarification, i replaced the air injector on the #6 after the second rebuild + another injector that was “flowing too much” with known good air injectors that were tested before given to me. The fuel injector checked out fine.

So was that a NEW piston? And how many hours before failure?

Yes that was the new piston, first rebuild lasted 4HR, second rebuild (pictures shown) lasted 9HR.

Melted #6 both times.

Measure the bore and piston diameter on all cylinders, match them so # 6 has the most clearance, .0065 to .007 measurements need to be taken at room temperature by a very skilled technician. Michs must be dead on accurate.
Did you follow the torque sequence on the powerhead mounting bolts to the letter? Note the lower tork values under #6
Was pcm put into break-in mode and was break-in procedure followed?
This is what I recomend with no deviation.
Attachment 523904

Curious of a comparison on the underside of piston crowns- say #5 and #6.

Attachment 523973
Attached are the pictures requested. I noted significantly more oil in the #5 compared to #6 cylinder. These are all pictures of #5cyl, today i checked the oil lines again and didn’t find anything out of the ordinary i blew air thru them and compared flow to other lines on the block.